# Differences

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+ | =====Near-rings===== | ||

+ | Abbreviation: **NRng** | ||

+ | ====Definition==== | ||

+ | A \emph{near-ring} is a structure $\mathbf{N}=\langle N,+,-,0,\cdot | ||

+ | \rangle $ of type $\langle 2,1,0,2\rangle $ such that | ||

+ | |||

+ | |||

+ | $\langle N,+,-,0\rangle $ is a [[groups]] | ||

+ | |||

+ | |||

+ | $\langle N,\cdot \rangle $ is a [[semigroups]] | ||

+ | |||

+ | |||

+ | $\cdot $ right-distributes over $+$: $(x+y)\cdot z=x\cdot z+y\cdot z$ | ||

+ | |||

+ | ==Morphisms== | ||

+ | Let $\mathbf{M}$ and $\mathbf{N}$ be near-rings. A morphism from $\mathbf{M}$ | ||

+ | to $\mathbf{N}$ is a function $h:M\rightarrow N$ that is a homomorphism: | ||

+ | |||

+ | $h(x+y)=h(x)+h(y)$, $h(x\cdot y)=h(x)\cdot h(y)$ | ||

+ | |||

+ | Remark: | ||

+ | It follows that $h(0)=0$ and $h(-x)=-h(x)$. | ||

+ | |||

+ | ====Examples==== | ||

+ | Example 1: $\langle\mathbb{R}^{\mathbb{R}},+,-,0,\cdot\rangle$, the near-ring of functions on the real numbers with pointwise addition, subtraction, zero, and composition. | ||

+ | |||

+ | |||

+ | ====Basic results==== | ||

+ | $0$ is a zero for $\cdot$: $0\cdot x=0$ and $x\cdot 0=0$. | ||

+ | |||

+ | ====Properties==== | ||

+ | ^[[Classtype]] |variety | | ||

+ | ^[[Equational theory]] |decidable | | ||

+ | ^[[Quasiequational theory]] | | | ||

+ | ^[[First-order theory]] | | | ||

+ | ^[[Locally finite]] |no | | ||

+ | ^[[Residual size]] |unbounded | | ||

+ | ^[[Congruence distributive]] |no | | ||

+ | ^[[Congruence modular]] |yes | | ||

+ | ^[[Congruence n-permutable]] |yes, $n=2$ | | ||

+ | ^[[Congruence regular]] |yes | | ||

+ | ^[[Congruence uniform]] |yes | | ||

+ | ^[[Congruence extension property]] | | | ||

+ | ^[[Definable principal congruences]] | | | ||

+ | ^[[Equationally def. pr. cong.]] | | | ||

+ | ^[[Amalgamation property]] | | | ||

+ | ^[[Strong amalgamation property]] | | | ||

+ | ^[[Epimorphisms are surjective]] | | | ||

+ | ====Finite members==== | ||

+ | |||

+ | $\begin{array}{lr} | ||

+ | f(1)= &1\\ | ||

+ | f(2)= &\\ | ||

+ | f(3)= &\\ | ||

+ | f(4)= &\\ | ||

+ | f(5)= &\\ | ||

+ | f(6)= &\\ | ||

+ | \end{array}$ | ||

+ | |||

+ | ====Subclasses==== | ||

+ | [[Rings]] | ||

+ | |||

+ | ====Superclasses==== | ||

+ | [[Groups]] | ||

+ | |||

+ | |||

+ | ====References==== | ||

+ | |||

+ | [(Ln19xx> | ||

+ | )] |

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